This invention relates to soft ethylenic polymer resin foams made from polymers, especially copolymers, of ethylene having relatively low modulus, i.e., materials lacking stiffness and which are limp, flexible and easily stretched. It particularly pertains to improvement in process whereby are obtained such soft foams having substantially closed cell structure and good dimensional stability.
The term "stiffness" as used herein means the property of plastics as determined by the test procedure described in ASTM D-747, expressed in pounds per square inch (psi).
It is well known to make closed cell ethylenic polymer resin foams by the process of extrusion foaming wherein a normally solid thermoplastic ethylenic polymer resin such as polyethylene is heat-plastified and mixed under pressure with a volatile material such as 1,2-dichlorotetrafluoroethane to form a flowable gel which is then passed through a shaping orifice or die opening into a zone of lower pressure. Upon the release of pressure, the volatile constituent of the gel vaporizes, forming a gas phase cellular structure in the gel which cools to a corresponding cellular foamed solid resin. Desirably, the resulting gas cells are substantially uniform in size, uniformly distributed through the foam body, and closed, i.e., separated from each other by membrane walls of resin. It is generally understood that the properties of the volatile material in context of the polymer resin and process technology affect the cell structure and properties of the foamed resin product. The volatile material should be sufficiently compatible with the resin under conditions of forming the flowable gel so that such gel is substantially homogeneous, but the solubility should be limited so that, when the pressure is released, the volatile material separates from the gel and forms gas bubbles or cells in the resin matrix. Further, the vapor pressure of the volatile material in the gel at the gel extrusion temperature should be high enough to exert pressure in the cell and on the cell walls causing the heat plastified resin to flow and the cellular foam to expand, but should not be so high as to cause the plastic cell walls to rupture which would allow the gas to escape without forming a cellular structure or would cause the bubbles to break and the foam to collapse. Although these and other general principles are thought to be understood, much of the extrusion foaming technology is empirical, based on experience, and directed to very specific materials and details to produce saleable products of narrowly defined specification.
One of the common requirements of acceptable foam resin products is dimensional stability, i.e., it is desired that the linear dimensions and thus the volume of a piece of foam resin not change appreciably, either to shrink or to expand, under ordinary conditions, from the time its manufacture is complete until the time its ultimate useful life is ended. It is also desired that if any appreciable shrinking of a foam is to occur, which is usually the case with a freshly extruded foam, the foam be able to recover within a reasonable period of time to a substantially constant volume close to that of the foam measured shortly after its extrusion. The difficulties of attaining dimensional stability are particularly acute in foams of relatively low density (high expansion ratio) when the resin membrane cell walls are relatively thin. It has been explained that the vapors of volatile material originally present in the cell gradually permeate the cell wall and escape from the foam over a period of time, thereby tending to reduce the inner cell pressure and tending to cause the foam to shrink during that time. However, when the foam is exposed to ambient atmosphere, air and its constituent gases also tend to permeate into the foam through the cell wall over a period of time thereby tending to increase the inner cell pressure. The difficulties of attaining dimensional stability are further accentuated in relatively thick foams. It has been observed that with such foams the time to reach substantially constant, commercially acceptable volume is relatively long, i.e., more time is required for rates of diffusion of residual blowing agent out of the foams and air into such foams to balance. Accordingly, the actual change in cell gas pressure and size is the result of complex and often opposite forces, and the resultant effect on resin foam dimensions is difficult to predict.
Although many volatile hydrocarbons, chlorohydrocarbons, fluorocarbons, and chlorofluorocarbons, as well as volatile ethers, ketones and other materials have been suggested for making extrusion foamed resin products, most are unsatisfactory in one or more respects when used individually. It has been suggested to use mixtures of two or more of such agents, or mixtures thereof with materials not useable alone, in attempt to compensate the inferior properties of each component with the superior properties of one or more other components, thereby to design a better foaming agent. In U.S. Pat. No. 3,766,099, for example, polyethylene is foamed by extrusion of a flowable gel containing a mixture of (A) dichlorodifluoromethane and (B) at least one of monochloropentafluoroethane and octafluorocyclobutane in certain proportions of (B) to (A) and optionally (C) one or more of certain aliphatic hydrocarbons and chlorofluorohydrocarbons. Under certain conditions, the gel is said to produce relatively stable foam products from polyethylene, whereas dichlorodifluoromethane alone produced foams showing considerable shrinkage on storage in air after production. However, the idea of using mixtures of volatile materials as blowing agents introduces even more complexity into the consideration of foaming behavior and makes prediction of results even more difficult.
These difficulties are even greater where, in place of polyethylene, there is used an ethylenic polymer resin having less stiffness, i.e., lower flexural modulus, than polyethylene, e.g., copolymers of ethylene and vinyl acetate (EVA) having stiffness (ASTM D-747) less than 20,000 psi. When such soft copolymers are used, the resulting foam is very sensitive to imbalances of rates of diffusion of the residual blowing agent out of the resin and out of the cells and air into such cells so that the tendency for dimensional instability, e.g., shrinking, is even greater than for stiffer resins and foams. For example, although U.S. Pat. No. 3,766,099 says that its process and mixed blowing agents can be used for foaming EVA resins in place of polyethylene, the fact is that, when the system preferred for use with polyethylene is used with a soft, low stiffness EVA resin, the resulting foam is dimensionally unstable and shrinks excessively on exposure to air, and there is no direction or instruction in the patent to correct the situation and to provide a satisfactory product.
Nevertheless, there is need and desire for foamed resin products which are softer and more flexible than conventional polyethylene foam products, especially for use in constructing items of wearing apparel, particularly for cushioning in sports equipment and athletic padding and for flotation in vests for water skiers, boating safety jackets and the like.
Accordingly, an object of this invention is to provide improved soft, flexible foamed resin products. Another object is to provide method and means for making such foam products. A particular object is to provide such improved method and means for making soft, flexible, substantially closed cell, low density foamed resin products from ethylenic polymer resins which have low stiffness, especially such foamed products having thickness greater than about 0.5 inch. Other objects and advantages of the invention are brought out in the description that follows.